1. Field of the Invention
The present invention relates to an optical element for correction of aberration, and a lithographic apparatus comprising same.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A lithographic apparatus includes a projection system. The projection system includes lenses which each may have aberrations. Such aberrations are typically small, but since device features to be imaged are becoming smaller with each new device generation, the relative influence of the aberrations is also increasing with each new device generation. Aberrations may be spherical and non-spherical. With increasingly smaller dimensions of features sizes, the non-spherical aberrations become of special concern.
These aberrations may be measured by a sensor called an “Integrated Lens Interferometer At Scanner” or briefly “ILIAS” which is an interferometric wavefront measurement system. ILIAS performs (static) measurements on lens aberrations up to Zernicke 36 as are needed for system setup and qualification. ILIAS is an on scanner integrated measurement system used for system setup and calibration. ILIAS is used for monitoring and recalibration of the scanner on a regular basis depending on the machine needs.